1. Field of the Invention
The present invention relates to radio receiver/transmitter filters having particular application to the problem of image rejection.
2. Description of the Related Art
Superheterodyne radio receivers are well known and amongst other things are characterised by the r.f. signal being applied to an i.f. stage which includes a mixer to which a local oscillator signal is applied. The products of mixing include the wanted signal and an unwanted image signal. Generally the image frequency is removed using L/C filters prior to mixing. However such filters do not readily lend themselves to fabrication as integrated circuits because of the need for an inductance. Although inductances can be simulated by gyrators it is difficult to obtain the required output power at VHF and UHF frequencies. In any event such filters do not lend themselves to accurate operation at high frequencies. In order to avoid this problem filter designers have attempted to use oscillators in filter circuits. W. Saraga in "Some Aspects of N-path and Quadrature Modulation Single-Sideband Filters" published in Network Theory, edited by R. Boite and published by Gorden and Breach discloses using quadrature modulation for single sideband generation. In one example two sets of quadrature mixers are used, each set being supplied with the same or different local oscillator signals. It has also been suggested by W. Saraga that the centre frequency of an N-path filter can be shifted away from the local oscillator frequency by means of feedback which will also increase the magnitude of the wanted signal relative to that of the unwanted image signal.
If these teachings of Saraga are used to provide an r.f. filter with its centre frequency moved away from the local oscillator frequency, then an additional mixer with its associated local oscillator would be required for mixing the signal to the i.f. The drawbacks to doing this are that not only would such an additional mixer involve more components but also mixers are power consuming parts of a circuit and furthermore they degrade the dynamic range of the circuit performance.